Transformer is one of the most critical components of a substation, whose safety, reliability and efficiency are of high importance to the overall power grid. For each transformer, especially power transformers with voltage level 110 kV and above, a dedicated cooling system consisting of multiple motor-fan units is required to keep the winding temperature within an acceptable range. The operation of the transformer is therefore closely related to 1) how the cooling system is designed and 2) how the cooling system is operated.
As to the cooling system design, it is common understanding that variable speed operation of these cooling fans can achieve higher efficiency compared with fixed speed operation. Therefore transformer cooling systems tend to install VFDs for motor-fan units to ensure high efficiency operation, the system architectures are shown in FIG. 1A and FIG. 1B. However, these two types of architecture have their own disadvantages. The first architecture as shown in FIG. 1A requires high capital investment because it installs VFD for each motor-fan chain; plus if the motor-fan chain is mostly working at rated speed, VFD solution might lower the efficiency due to its own power losses. The second architecture as shown in FIG. 1B can relatively reduce the capital investment because it uses one big VFD to drive a plurality of motor-fan chains jointly at the same operation point. But the disadvantages are also obvious: Firstly, each motor-fan chain has low efficiency when the VFD utilized capacity is relatively low; secondly, there are different ways for load distribution among different VFD-fed motor-fan chains to meet the same total output requirement. It is not always true to distribute the load evenly among individual chains in order to have optimal system efficiency.
As to cooling system operation, the core is how to control the winding temperature. Normally, lower winding temperature leads to the lower copper loss of winding. However, the power consumption of the cooling system will be higher at the same time, meaning that the overall efficiency, considering both transformer winding and the cooling system itself, might be less optimal.
Besides efficiency, the variation of the winding temperature is also one key factor which will affect the lifecycle of the transformer. The more frequency the temperature varies, the faster the transformer aging will be. It could be so that the efficiency of the transformer is optimized, however at a cost of shortened transformer lifetime.
For transformer operated at urban area, noise level is also one important criterion to consider in order to reduce the impact on the neighbouring residents especially at night. Currently, few solution is available to control the cooling system to tackle the noise problem.
To overcome above shortcomings, the person skilled in the art aims to solve two problems as follows.
1) How to design the cooling system to realize speed regulation for the motor-fan loads selectively with less capital investment on VFDs.
2) How to improve the operation efficiency of transformer by cooling control considering the transformer copper loss, the motor-fans power consumption and the speed regulation of VFD.
3) How to control the winding temperature as well as its variation in order to extend the lifecycle of the transformer and meanwhile achieve the best overall system efficiency.
4) How to operate the cooling system to optimize not only the efficiency and lifecycle, but also minimize the noise level so as to reduce the negative impact on the surrounding environment.